In vehicles having an internal combustion engine, the exhaust gas temperature is generally held within definite temperature ranges for several reasons: to avoid thermal overload of the component parts in the exhaust gas system; to reach or maintain the operating temperature (lightoff) of catalytic converters in the exhaust branch; and possibly also to bring an NOx-storing catalyst to a high temperature so as to discharge stored sulfur.
In order to heat up catalytic converters in the exhaust branch, the ignition firing point is often delayed. For this reason, combustion does not proceed optimally. For equal torque produced, exhaust gas temperatures are then higher.
Thermal overload of the component parts in the exhaust gas system is usually prevented by the following interventions:
The most retarded ignition angle is limited as a function of the operating point of the engine. For the purpose of engine heating of the catalytic converter, or on account of a torque aiming-off allowance for rapid controller interventions, the ignition angle may lie more retarded in time than the optimum ignition angle for maximum torque and power efficiency.
For a short-time retarding of ignition, the combustion limit is a deciding factor. For continuing retarded ignition, the maximum ignition angle allowable is the angle at which critical temperatures have not yet been reached in the exhaust manifold. This retard limit is ascertained on the basis of an operating state of the engine. In this connection, the operating state is defined, for instance, by values for air charge, engine output and engine speed.
This intervention is relevant particularly for the partial throttle range. It may be seen as a passive restriction for the protection of component parts.
A further possibility for intervention to prevent thermal overload is to enrich the mixture as a function of modeled or measured temperatures in the exhaust gas system.
If the modeled/measured temperatures in the exhaust gas system exceed critical limits, rich engine operation is employed.
The exhaust gas is cooled by the enthalpy of vaporization of the excess fuel. This intervention is relevant particularly for operating points near full throttle. It may be viewed as an active intervention.
For a heating phase directly following start-up, limitation of the ignition angles is sufficient protection of component parts since the exhaust tract is still cold. Because of that, thermal damage can also be discounted even when the exhaust gas temperatures are excessively high for a short time period. In the case of engines in lean-combustion operation having an NOx-storing catalyst (e.g. engines having direct fuel injection), the NOx-storing catalyst is cyclically desulfurized. For this, the NOx-storing catalyst is heated to temperatures above 600° C. during operation. In this connection, critical temperatures in the exhaust branch may be reached in the partial throttle range.
Alternatively, a leaner engine operation typically requires the NOx-storing catalyst to be within a temperature range of 200° C.-450° C. The exhaust gas system is normally designed so that these temperatures are ensured over a wide operating range. Therefore, measures for cooling the exhaust gas may be necessary. The most simple variant is to let the exhaust branch have travel air flowing over it to increase the heat loss by convection.
For a heating phase directly following start-up, limitation of the ignition angles is sufficient protection for component parts because the exhaust tract is still cold. Because of that, thermal damage can also be discounted in case the exhaust gas temperatures are too high for a short period. For the desulfurization of an NOx-storing catalyst, the latter is heated to temperatures above 600° C. during normal engine operation. In this case, critical temperatures in the exhaust branch may be reached, especially in the part throttle range. This problem can become more stringent on account of the cooling measures for the NOx-storing catalyst described above. Component parts positioned upstream of the catalytic converter can then approach near their critical temperature.
Conventionally, interventions for component part protection which have the purpose of limiting exhaust gas temperatures, and interventions for heating up the catalytic converters which have the purpose of raising exhaust gas temperatures, have been performed independently of one another.
This may be problematic, because heating measures may affect measures for exhaust gas cooling. If, for example, because of retarded ignition angles, the air-fuel mixture is enriched, an inefficiency arises if the critical temperatures are not attained because of the limitation on the ignition angles. In addition, emissions may be worsened.
In order to maintain operating temperatures of the NOx-storing catalyst over wide operating ranges, further active exhaust gas cooling measures may be applied such as controllable incident flow to the exhaust branch via damper, and rerouting of the exhaust gas via heat exchanger. These cooling measures may make enrichment unnecessary.